We present the Measurement Manager Protocol (MGRP),
an in-kernel service that schedules and transmits probes
on behalf of active measurement tools. Unlike prior measurement
services, MGRP transparently piggybacks application packets inside
the often significant amounts of empty padding contained in typical
probes. Using MGRP thus combines the modularity, flexibility, and
accuracy of standalone active measurement tools with the lower
overhead of passive measurement techniques. Microbenchmark experiments
show that the resulting bandwidth savings makes it possible to
measure the network accurately, but faster and more aggressively than
without piggybacking, and with
few ill effects to piggybacked application or competing
traffic. When using MGRP to schedule
measurements on behalf of MediaNet, an overlay service
that adaptively schedules media streams, we show MediaNet can achieve
significantly higher streaming rates under the same network
conditions.